1. Field of the Invention
The present invention relates to dielectric filters, dielectric duplexers, and communication apparatuses incorporating the same.
2. Description of the Related Art
A coaxial composite dielectric filter is a conventional type of bandpass filter used in microwave bands. The coaxial composite dielectric filter is formed by arranging a plurality of resonance line holes having resonance lines formed on the inner surfaces thereof in a dielectric block and forming an outer conductor on the outer surfaces of the dielectric block.
Particularly, Japanese Unexamined Patent Application Publication No. 2-92001 discloses a dielectric filter in which the inner diameter of each of the resonance line holes is changed at a position in the axial direction of each resonance line hole to form a stepped part.
For example, a conventional dielectric filter having a stepped part formed by changing the inner diameter of each of the resonance line holes is shown in FIG. 13. FIG. 13 shows a perspective view of the dielectric filter, in which the top surface is the surface used when the dielectric filter is mounted on a circuit board. In this figure, the reference numeral 1 denotes a substantially rectangular-parallelepiped dielectric block, inside which resonance line holes 2a and 2b are formed. The resonance line holes 2a and 2b are through-holes penetrating two substantially parallel opposing surfaces of the dielectric block 1. The inner diameter of each of the through-holes is changed at a specified position in the axial direction of each of the holes to form a stepped part. An inner conductor is disposed on the inner surface of each of the resonance line holes 2a and 2b to form a respective resonance line. In addition, an outer conductor 3 is disposed on five surfaces of the dielectric block 1 and connected to the resonance line holes 2a and 2b except on one of the surfaces where the resonance lines are open-circuited. On outer surfaces of the dielectric block 1, terminal electrodes 4a and 4b separated from the outer conductor 3 are formed. A capacitance is formed between the terminal electrodes 4a and 4b and parts near the open-circuited ends of the resonance lines to make capacitive coupling.
In this way, in the dielectric block 1, one of the opening ends of each resonance line hole is a short-circuited end, and the other opening end thereof is an open-circuited end, to constitute a xc2xc-wavelength resonator.
In the above dielectric filter, while maintaining the axial length of each of the resonance line holes fixed, the resonance frequency of a resonance element formed by each resonance line hole can be adjusted to a desired value.
However, although the strength of the capacitive coupling between the adjacent resonators can be adjusted by changing the position of the stepped part in the axial direction, it is impossible to change the capacitive coupling to inductive coupling, that is, it is impossible to change the polarity of coupling.
Accordingly, the present invention provides a dielectric filter and a dielectric duplexer having coupling structures between resonators, in which the variable range of coupling strength is broadened and the polarity of coupling can be changed. The present invention further provides a communication apparatus incorporating one of the dielectric filter and the dielectric duplexer.
According to a first aspect of the invention, there is provided a dielectric filter including a plurality of resonance lines substantially parallel to each other arranged on an upper surface of a dielectric substrate or inside a dielectric block, and an outer conductor formed on the lower surface of the dielectric substrate or on the outer surface of the dielectric block. In the dielectric filter, both ends of each of the plurality of resonance lines or parts near both ends thereof are open-circuited, and the width of at least one of the resonance lines is changed in at least one position in the longitudinal direction of the resonance line to form a stepped part.
According to a second aspect of the invention, there is provided a dielectric duplexer including a pair of dielectric filters of the type described above, formed on a single dielectric plate or inside a single dielectric block to be used as a transmitting side filter and a receiving side filter, respectively. A transmitted-signal input terminal is coupled to a first-stage resonance line of the transmitting side filter, a received-signal output terminal is coupled to a last-stage resonance line of the receiving side filter, and an antenna terminal is coupled in common to both a last-stage resonance line of the transmitting side filter and a first-stage resonance line of the receiving side filter.
In addition, according to a third aspect of the invention, there is provided a communication apparatus including one of the dielectric filter and the dielectric duplexer, which may be used either as a filter or as a duplexer for transmitting/receiving signals in a high-frequency circuit.
The foregoing and other features and advantages of the invention will be appreciated from the following detailed description of embodiments thereof, with reference to the drawings, in which like references denote like elements and parts.